1. Field of Invention
The subject matter of the present invention relates generally to a metal detector which employs an electromagnetic field produced by a transmit coil to induce a received signal in the receive coil when a meal object is within the portion of the electromagnetic field external to the coils, and more particularly to a metal detector wherein the received signal is sampled and the sample voltage utilized to produce an output signal corresponding to a selected component of the received signal.
Possibly the greatest problem in the use of existing transmit-receive type metal detectors is their annoying generation of signals in the receive coil due to mineral soil conditions. These signals produced by soil conditions often make the detection of a small metal object, such as a coin, in areas having highly mineralized and changing soil conditionsimpossible. This problem is solved in a straighforward, simple, and inexpensive manner by using sampling to produce the output signal of the metal detector in accordance with the present invention. Thus the received signal induced in the receive coil by the metal object is sampled to produce an output voltage that ignores the changes in the received signal arising from soil conditions. For example, in a mineral soil which affects only the reactive component of the induced received signal, sampling is done at a time so that the output excludes any signal portion due to such reactive component and the changes in the reactive component caused by changes in soil conditions will be ignored.
2. Description of the Prior Art
Previously known metal detectors do not utilize a sampling technique to detect the presence of metal objects by producing an output voltage representing a selected component of a signal induced in a receive coil. An eddy current testing device, disclosed in U.S. Pat. No. 3,337,796 of Hentschel, does use a sampling technique, but does not in any manner suggest that this could be employed to detect the presence of previously undetected metal objects as required by a metal detector. In Hentschel, a metal bar or other specimen to be tested is inserted into the center of a primary coil and of a secondary coil so that such specimen directly couples the primary coil to the secondary coil in order to produce an output signal from the secondary coil. Unlike this eddy current tester, in the metal detector of the present invention the received signal is induced in the received coil when a metal object is present within an electromagnetic field external to the transmit and receive coil. Thus, such eddy current tester cannot be used as a metal detector to detect previously unknown buried metal objects because these objects cannot be inserted within the primary and secondary coils.